420 EIGHTH PACIFIC SCIENCE CONGRESS 



the Atlantic around the entrance to the Mediterranean and the Red 

 Sea through the Indian Ocean, the West Pacific, Southwest Pacific and 

 the Central Pacific, is a good evidence that these particular species were 

 already existing in their present habitats long before the formation of 

 the land barrier between the two American continents and that of Eu- 

 rope and Africa. With the land barriers now existing, it is hard to 

 explain their circumtropical distribution through their pelagic auricula- 

 rian larvae by way of the northern and southern tips of the different 

 continents. It is more logical to believe the former existence of the 

 Tethys Sea which connected the Indo-Pacific and the Atlantic oceans 

 up to the Miocene of the Tertiary Period. 



The intermigration of similar or identical species of holothurians 

 within the Indo-Pacific area through their auricularian larvae is a pos- 

 sible explanation of the wide distribution of many of the species listed. 

 Mortensen (1925) reported that in his observation on Stichopus cali- 

 forniciis (Stimpson), the auricularian larvae after the third week still 

 remained for some time without much differentiation. It is safe to 

 speculate, therefore, that an auricularian larva may remain pelagic for 

 some time before they settle down to start their sedentary life, hence 

 it is possible for them to be carried by the current to distant places 

 usually in an east to west direction. This also explains why there are 

 more species of holothurian listed in the West Pacific, Southwest Paci- 

 fic, and the Indian Ocean, which fact confirmed the findings of others 

 that the Indo-Malayan region is the richest in marine life species. 



Comparing the holothurian fauna known from the Atlanto-East- 

 Pacific region with those of the Indo-West-Pacific, two distinct differences 

 may be noted, namely, the entirely different species of the fauna and the 

 predominance of the dendrochirote holothurians over the aspidochirotes. 

 Although there are few species that are almost bipolar in distribution, 

 yet the few known species from the western coast of South America are 

 endemic to the place. The distinct differences of the holothurian fauna 

 of the Southeast Pacific from that of the oceanic islands of the Central 

 Pacific may be explained by the deep ocean barrier between them and 

 the upwelling of cold bottom water along the southwestern coast of 

 South America. 



Among the few dendrochirotes from the southern tip of South 

 America, Psolus squamatus (Koren) var. segregatus Perrier is also found 

 in the Bering Sea of North America. The absence of this species in 

 the East Pacific makes it a bipolar form and its bipolar distribution 

 may be explained by the Relict Theory of Theel, Pfeiffer and Murray. 

 This species apparently was formerly a cosmopolitan warm-water form 

 and became extinct in the tropical regions for unknown reasons, or it 



